I imagine it depends on how seriously mail readers take the charset
declaration in the header.
Pegasus Mail, which I use at home for somethings, seems to ignore it
completely, for example, and treats ascii, iso-8859-1, iso-8859-7
(Greek), utf-8, and anything else for that matter identically: as the
current Windows character set (code page 1252): that is, it ignores
the charset.
To read conlang, I use Gmail over Firefox, and I think Firefox is more
picky, so it shows question marks if a byte arrives in a message that
is illegal according to the declared character set.
I imagine Outlook Express will display the characters "correctly"
(read: as you intended), since MS is not known for caring much about
the charset declaration (e.g. sending out Windows-1252 messages with
"smart quotes", em dashes, and the like as iso-8859-1).

> I also downloaded Mozilla, but I don't completely
> master it yet. Also, I'd like to get list messages in
> one mailbox, and others in another one, that's the
> only reason why I'm using Yahoo at the time.

Consider using Fastmail.FM for conlang then :) It'll be a separate
mailbox for list messages. Or Gmail, if you can get hold of it - then
you'll have messages in one conversation kept together, which can be
quite handy indeed in conlang, which often has multiple lengthy
conversations going on in parallel.
Cheers,
--
Philip Newton <philip.newton@...>